They also bring up an interested paradox in regards to researching the genealogy of Disney characters:

Building a family tree of a Disney character is not an easy job : first, you have to include all the concerned characters, which is somehow difficult, because you have to lead huge researches, then you have to face contradictions which exist from a story to another, from an “universe” to another,… But the most delicate issue is the big lack of normal family relations in the Disney universe (we can find as many uncles, aunts, nephews and nieces as possible, but direct parents or children are very rare). About this relevant topic, Don Rosa told “I don’t really know the reason for there never being fathers and mothers, only aunts and uncles, in Disney comics. Maybe it makes the kids possible but doesn’t limit the actions of the “parents”… Unca Donald can act slightly less responsible than a real father, with no mother to watch him. With the uncle/aunt deal you have all story possibilities still open with no limitations.”